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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

POZZOLANE AND SLAGS

149

results given graphically in a diagram (days as abscissae and penetration of
the needle in mm. as ordinates).

In the second case, that is with powdered pozzolane, the mortar is pre-
pared from i part of powdered slaked lime and 4 parts of the pozzolana,
these being mixed first dry and then with water; the consistency
of the mortar and the setting are tested by methods indicated later for
cements*

7. Strength.—Tensile and compression tests are carried out as with
cements.

With a granular pozzolana, the moulds are filled by hand by means of
a spatula with the mortar prepared as for setting tests ; after some days,
when the briquettes have attained a certain consistency, they are removed
from the moulds and kept first in the moist chamber until 7 days after
mixing and subsequently some in water and some in moist air. The strength
test is made after 28 days and may be repeated after 84 days, 210 days
and i year.

In the case of powdered pozzolana, a normal mortar is prepared from
[ part of the lime-pozzolana mixture (i of lime and 4 of pozzolana) and
3 parts of normal sand (see later: Cements), the procedure for rilling the
moulds and carrying out the tests being the same as for cements.

*

* *

The chemical compositions of some of the better known pozzolane, santorins
and trass are given in Table IV.

Pozzolane should be free from extraneous and inert substances and for the
measurements, both by weight and by volume, they should not contain more
than 10% of moisture.

Pozzolane are regarded as energetic when, in the natural granular state, they
answer the following requirements : (i) the normal mortar (3 parts by weight
of pozzolana to i of lime), after 7 days in the moist chamber, does not allow the
Vicat needle weighing i kilo to penetrate more than 7 mm. when falling from a
height of 30 mm.; (2) after 28 days—7 in moist air and 21 in water—the briquettes
of normal mortar exhibit a tensile strength of not less than 4 kilos per sq. cm.
and a compression strength of at least 20 kilos per sq. cm.

Feeble pozzolane are those yielding normal mortars which do not attain
the above strength but allow, after 7 days, the needle to penetrate not more than
10 mm., and after 28 days exhibit tensile and compressive strengths of 2 and 10
kilos per sq. cm. respectively. If these standards are not attained, the material
is not regarded as pozzolanic in character.

Slags, to be of use in the cement industry, should be basic, that is, the ratio
CaO : SiO2 should be not less than i, and they should be as rich as possible in
alumina and as poor as possible in manganese, magnesia and sulphides. The
composition of a good slag should lie between the limits : SiO2, 25-36% ; A1203,
10-22% ; Fe2O3, up to 1-5% ; FeO, up to 2% ; MnO, up to 3% ; CaO, 30-50% ;
MgO, up to 3% ; CaSO4, up to 2% ; and CaS, up to 3%. Alkalies are usually
present in small quantity.eter 3-2 mm. at the, base and H>(> mm. at the apex. The needle